1. Field of the Invention
The present invention relates to a semiconductor device, such as a thin-film transistor, having in its main part a polysilicon thin film for transferring carriers.
2. Description of the Related Art
A technique of forming a thin-film transistor on an insulating substrate is known in the field of active matrix liquid crystal displays (LCDs). The technique allows a semiconductor integrated circuit to be formed on a transparent insulating substrate formed of, for example, glass, or a large insulating substrate which cannot be formed of a monocrystalline semiconductor. At present, a thin-film transistor generally includes an active layer made of amorphous silicon or polysilicon. Since amorphous silicon can be formed at a low temperature, it is applicable to an active matrix LCD which must be formed on a glass substrate having a low melting point.
However, since amorphous silicon has a low electron mobility, it has been used only as a switching element for charging a pixel capacitor electrode to apply an electric field to a liquid crystal. An active layer made of polysilicon must be used in a circuit element, e.g., a driver circuit, a ROM, a RAM or a CPU, which must be driven at a high speed. For example, a driver circuit of a high-quality, large-screen liquid crystal TV or a high-definition office automation (OA) liquid crystal display panel is driven at a clock frequency of about 10 MHz. If such a driver circuit is formed of a semiconductor device, a field effect mobility .mu..sub.FE of 50 cm.sup.2 /V.sec or more, preferably 80 cm.sup.2 /V.sec or more is required. However, the field effect mobility .mu..sub.FE of a polysilicon thin-film transistor known to the public at present is at most 30 cm.sup.2 /V.sec.